1. Field of Invention
The present invention relates to method and apparatus for reducing data rate of image pixels, and particularly relates to compression and decompression of the temporary image frame with variable bit rate ranging from frame to frame.
2. Description of Related Art
The digital image and motion video have been adopted in an increasing number of applications, which include digital camera, scanner/printer/fax machine, video telephony, videoconferencing, video phone, VCD (Video CD), DVD, digital TV . . . etc. The success of development of the digital image and video compression standards including JPEG, a still image compression standard set by ITU and the MPEG, the motion video compression standard set by the ISO have played key role for the digital image and video applications.
Both JPEG and MPEG are lossy algorithms which when being decompressed, the image data are different from the original ones. And the bit rate of each image frame is variable depending on the complexity of the image pattern.
In the LCD TV, LCD monitor, LCD Driver, DVD, STB, Set-Top-Box, 3G mobile phone or other portable devices like scanner/printer, a decompressed video stream of continuously image will be temporarily saved into a storage device for other manipulation including scaling, de-interlacing and frame rate conversion before being sent to a display device or saving to a storage device. In these kind of applications, a fixed bit rate of compressed temporary image frame has convenience of allocating fixed density of the storage device or/and allocating fixed bandwidth of accessing the storage device during a pre-scheduled time slot. Fixed bit rate of a compressed temporary image frame provides necessary performance of quick accessing pixels of any image frame in the storage device which results in the saving of cost and power consumption. Since these image frames temporarily saved in the storage device will be used as references in the further image frames, keeping good quality is required for ensuring the good image quality for next image.
The main disadvantage of applying fixed bit rate in compressing a temporary image frame is that it is difficult to guaranty the image quality for each of image frames since the image pattern complexity varies widely from frame to frame.
Therefore, it is beneficial to reduce the bit rate of an image frame with a predetermined fixed bit rate while still keeping good image quality for potential adjustment in compression ratio from frame to frame. This invention applies variable compression ratio and mechanism of inserting starting address of each compressed image frame for quick random accessing each frame and each line within each frame of image.